High power switching transistors are sensitive to turn-off conditions, particularly when operating in the presence of an inductive load. Inductive loading tends to create high instantaneous power dissipation in the transistors during switching because the locus of the operating point defines a path in the (I.sub.c V.sub.ce) plane that is significantly placed from the origin. In order to increase turn-off speed, which is necessary to minimize turn-off power dissipation, a negative bias is generally applied to the base terminal of the transistor (power transistors are typically of the NPN type) resulting in reverse current drive. This negative bias promotes current crowding causing local hot spots, second breakdown and burnout. Transistor turn-off during a system fault is particularly difficult because the transistors may be conducting abnormally high current during turn-off. It is a general object of the present invention to reduce the current flow through a power switching transistor during turn-off in the presence of load current overload to reduce turn-off power dissipation.
When a transistor having a inductive load is turned off, the sudden reduction of current flowing through the inductive load causes its magnetic flux field to collapse. The collapsing flux produces a back electromotive force (EMF) of voltage across the inductor at such a polarity as to generate a current to oppose the changing flux. In high magnitude loads of the type commonly encountered in motor control, for example, and switching times on the order of one microsecond or less, the back EMF applied across the transistor may be on the order of magnitude of 100's of kilovolts at high instantaneous current levels.
Voltage snubber circuits connected in shunt with the load protect the switching transistors during turn-off from high inductive load generated surge voltages by providing a flow path to ground for current generated by the back EMF of the inductive load. The snubber capacitor is charged by the voltage supply through the switching transistors during the turn-on periods and discharged through the inductive load during the turn-off periods to provide the snubbing. During each turn-on period, the snubber capacitor charges to the level of the voltage generated by the switching transistors to be discharged through the load during the subsequent turn-off period. Generally, due to the time constant inherent in the RC snubber circuit, the time required to fully charge the snubber capacitor is about 150 microseconds, depending upon particular resistance and capacitance values that are a function of the operating frequency of the inverter, and load current levels. During normal operation, the snubber capacitor always becomes fully charged prior to a subsequent turn-off period since the turn-on periods are much greater than 150 microseconds. In the event of a fault condition occurring during the 150 microsecond time period following transistor turn-on, the snubber circuit is inoperative so that the transistor is unprotected from voltage surges during turn-off by an overload detector. Another object of the present invention, therefore, is to provide auxiliary snubbing during the inoperative period inherent in the conventional snubber circuit at the start of the transistor turn-on transitions.